The present invention generally relates to electro-optic devices, and more specifically relates to rearview mirrors of a vehicle.
To enable water droplets and mist to be readily removed from the windows of a vehicle, the windows are typically coated with a hydrophobic material that causes the water droplets to bead up on the outer surface of the window. These water beads are then either swept away by windshield wipers or are blown off the window as the vehicle moves.
It is equally desirable to clear external rearview mirrors of water. However, if a hydrophobic coating is applied to the external mirrors, the water beads formed on their surfaces cannot be effectively blown off since such mirrors are relatively shielded from direct airflow resulting from vehicle movement. Thus, water droplets or beads that are allowed to form on the surface of the mirrors remain on the mirror until they evaporate or grow in size until they fall from their own weight. These water droplets act as small lenses and distort the image reflected to the driver. Further, when the water droplets evaporate, water spots are left on the mirror, which are nearly as distracting as the water droplets that left the spots. In fog or high humidity, mist forms on the surfaces of the external mirrors. Such a mist can be so dense that it effectively renders the mirrors virtually unusable.
In an attempt to overcome the above-noted problems, mirror manufacturers have provided a hydrophilic coating on the outer surface of the external mirrors. See U.S. Pat. No. 5,594,585. One such hydrophilic coating includes a single layer of silicon dioxide (SiO2). The SiO2 layer is relatively porous. Water on the mirror is absorbed uniformly across the surface of the mirror into the pores of the SiO2 layer and subsequently evaporates leaving no water spots. One problem with such single layer coatings of SiO2 is that oil, grease, and other contaminants can also fill the pores of the SiO2 layer. Many such contaminants, particularly hydrocarbons like oil and grease, do not readily evaporate and hence clog the pores of the SiO2 layer. When the pores of the SiO2 layer become clogged with car wax, oil, and grease, the mirror surface becomes hydrophobic and hence the water on the mirror tends to bead leading to the problems noted above.
A solution to the above problem pertaining to hydrophilic layers is to form the coating of a relatively thick layer (e.g., about 1000–3000 Å or more) of titanium dioxide (TiO2). See European Patent Application Publication No. EPO 816 466 A1. This coating exhibits photocatalytic properties when exposed to ultraviolet (UV) radiation. More specifically, the coating absorbs UV photons and, in the presence of water, generates highly reactive hydroxyl radicals that tend to oxidize organic materials that have collected in its pores or on its surface. Consequently, hydrocarbons, such as oil and grease, that have collected on the mirror are converted to carbon dioxide (CO2) and hence are eventually removed from the mirror whenever UV radiation impinges upon the mirror surface. This particular coating is thus a self-cleaning hydrophilic coating.
One measure of the hydrophilicity of a particular coating is to measure the contact angle that the sides of a water drop form with the surface of the coating. An acceptable level of hydrophilicity is present in a mirror when the contact angle is less than about 30°, and more preferably, the hydrophilicity is less than about 20°, and most preferably is less than about 10°. The above self-cleaning hydrophilic coating exhibits contact angles that decrease when exposed to UV radiation as a result of the self-cleaning action and the hydrophilic effect of the coating. The hydrophilic effect of this coating, however, tends to reverse over time when the mirror is not exposed to UV radiation.
The above self-cleaning hydrophilic coating can be improved by providing a film of about 150 to 1000 Å of SiO2 on top of the relatively thick TiO2 layer. See U.S. Pat. No. 5,854,708. This seems to enhance the self-cleaning nature of the TiO2 layer by reducing the dosage of UV radiation required and by maintaining the hydrophilic effect of the mirror over a longer period of time after the mirror is no longer exposed to UV radiation.
While the above hydrophilic coatings work well on conventional rearview mirrors having a chrome or silver layer on the rear surface of a glass substrate, they have not been considered for use on variable reflectance mirrors, such as electrochromic mirrors, for several reasons. A first reason is that many of the above-noted hydrophilic coatings introduce colored double images and increase the low-end reflectivity of the variable reflectance mirror. For example, commercially available, outside electrochromic mirrors exist that have a low-end reflectivity of about 10 percent and a high-end reflectivity of about 50 to 65 percent. By providing a hydrophilic coating including a material such as TiO2, which has a high index of refraction, on a glass surface of the mirror, a significant amount of the incident light is reflected at the glass/TiO2 layer interface regardless of the variable reflectivity level of the mirror. Thus, the low-end reflectivity would be increased accordingly. Such a higher low-end reflectivity obviously significantly reduces the range of variable reflectance the mirror exhibits and thus reduces the effectiveness of the mirror in reducing annoying glare from the headlights of rearward vehicles.
Another reason that the prior hydrophilic coatings have not been considered for use on many electro-optic elements even in applications where a higher low-end reflectance may be acceptable or even desirable is that they impart significant coloration problems. Coatings such as those having a 1000 Å layer of TiO2 covered with a 150 Å layer of SiO2, exhibit a very purple hue. When used in a conventional mirror having chrome or silver applied to the rear surface of a glass element, such coloration is effectively reduced by the highly reflective chrome or silver layer, since the color neutral reflections from the highly reflective layer overwhelm the coloration of the lower reflectivity, hydrophilic coating layer. However, if used on an electrochromic element, such a hydrophilic coating would impart a very objectionable coloration, which is made worse by other components in the electrochromic element that can also introduce color.
Another reason that prior art coatings have not been considered for use on many electro-optic elements is haze. This haze is particularly evident in hydrophilic coatings comprising dispersed TiO2 particles in a binding media such as SiO2. Titanium dioxide particles have a high refractive index and are very effective at scattering light. The amount of light scattered by such a first surface hydrophilic coating is small relative to the total light reflected in a conventional mirror. In an electro-optic mirror in the low reflectance state, however, most of the light is reflected off of the first surface and the ratio of scattered light to total reflected light is much higher, creating a foggy or unclear reflected image.
Due to the problems associated with providing a hydrophilic coating made of TiO2 on an electrochromic mirror, manufacturers of such mirrors have opted to not use such hydrophilic coatings. As a result, electrochromic mirrors suffer from the above-noted adverse consequences caused by water drops and mist.